1. Field of the Invention
This invention relates to the field of processes for polymerizing vinyl ester-alkyl acrylate/methacrylate comonomers.
2. Description of the Prior Art
Vinyl-acrylic copolymers are commonly prepared by way of emulsion polymerization techniques. A typical procedure involves admixing under rapid stirring the comonomer charge with water, surfactants or emulsifying agent, a polymerization catalyst or initiator, and, in most cases, a protective colloid-forming substance. The mixture is then heated to polymerization temperature for the time required to substantially complete the emulsion polymer formation. Many variations of the general preparative procedure are used to obtain, for example, lower viscosity, high solids content, improved freeze-thaw stability, etc. These variations include, for example, the use of a pre-emulsion technique (see U.S. Pat. No. 3,637,563) in which the monomer charge is first emulsified in water using a surfactant or emulsifying agent, and then the pre-emulsion is added to the reaction mixture which contains the remaining reactants; intermediate addition of surfactant to the polymerization reaction mixture during the polymerization (see U.S. Pat. No. 3,423,352); and addition of a water-soluble free radical initiator to a substantially completely polymerized emulsion and reacting further to reduce the viscosity of the emulsion (see U.S. Pat. No. 3,423,353).
According to commonly assigned U.S. Pat. No. 4,056,503 issued Nov. 1, 1977, a comonomer charge of vinyl acetate and an alkyl acrylate, including a multifunctional acrylate, is added to an aqueous surfactant-containing solution to provide a pre-emulsion which is then added to a polymerization reaction medium containing a redox catalyst and pre-heated to reaction temperature. This and similar pre-emulsion processes require special equipment and constant monitoring if substantially grit-free polymerizates are to be obtained. Accordingly, a thermally initiated emulsion polymerization procedure which does not require the preparation of a pre-emulsion would be technically and economically attractive if the resulting latex copolymer were free of coagulum and otherwise met the standards of the conventionally produced product. Actual attempts to carry out such a procedure employing a comonomer charge containing a multifunctional acrylate were unsuccessful, possibly because of the high rate of reactivity of the multifunctional comonomer. In the course of polymerization, coagulum formation developed at a fairly early stage and coatings of the unpigmented lattices were gritty and opaque.